1. Field of the Invention
The present invention relates to a vacuum carburization method and a vacuum carburization apparatus.
2. Description of the Related Art
Vacuum carburization is a carburization technique in which a surface layer of a metallic workpiece is carburized and quenched to enhance the hardness of the surface layer. Such vacuum carburization is described in Japanese Patent Application Laid-Open No. 8-325701 (hereinafter, referred to as Patent Document 1) and Japanese Patent Application Laid-Open No. 2004-115893 (hereinafter, referred to as Patent Document 2).
In the vacuum carburization described in Patent Document 1, a workpiece is vacuum-heated in a heating chamber to a predetermined temperature, carburizing gas such as acetylene is supplied into the heating chamber, and the workpiece is carburized. Then, the supply of the carburizing gas is stopped, the inside of the heating chamber is made into the vacuum state again, carbon on the surface of the workpiece is diffused thereinto, the temperature is lowered to a quenching temperature, and then an oil cooling is performed.
In the vacuum carburization described in Patent Document 2, in order to solve excessive carburization of a surface (particularly, a corner) of a workpiece, in the initial stage of the diffusion in the vacuum carburization described in Patent Document 1, a decarburizing gas is introduced into a furnace (equivalent to the heating chamber described in Patent Document 1), so that cementite in the surface layer of the workpiece is reduced or removed.
FIGS. 12 and 13 are diagrams illustrating a treatment time and a temperature of each process of the conventional vacuum carburization, an atmosphere condition, and examples of apparatus types when a ring gear for automobile is processed. In this processing, a steel material such as SCr420 with a basic material carbon concentration of 0.2% is used as a workpiece, a target surface carbon concentration is 0.8%, an effective carburization depth is 0.8 mm in FIG. 12 and 1.5 mm in FIG. 13, and a target carbon concentration at the effective carburization depth is 0.35%.
In the conventional vacuum carburization described above, as shown in FIGS. 12 and 13, after the diffusion process, the temperature is lowered to the quenching temperature in the temperature lowering process, and then the process is transferred to a maintaining process before quenching. In this case, generally the carburization temperature X° C. is about 930° C. Since the rate of the carburization and the diffusion gets higher as the treatment temperature gets higher, it is possible to shorten the time for the vacuum carburization.
However, when the vacuum carburization is performed, for example, at the treatment temperature X° C. of 1050° C., it is difficult to form fine crystal grains in the workpiece W due to bloating caused by the high temperature treatment. Accordingly, it is difficult to obtain a workpiece W having a predetermined property value. In addition, non-uniformity in temperature occurs between the surface and the inside of the workpiece and thus the crystal grains become non-uniform.
The invention has been made to solve the aforementioned problems, and an object of the invention is to raise the treatment temperature to allow rapid progress of the carburization and the diffusion, achieve uniformity in temperature between the surface and the inside of the workpiece even when the treatment time is shortened, and to solve the problem of the crystal grains being bloated, thereby obtaining a workpiece having a predetermined property value.